What do the Kesset need with all those slaves?

you may ask.

The Ta'arane cities along the foot of the escarpment harnessed a huge amount of kinetic power in the form of water flowing downhill. Mills to grind grain and decorticate flax, water-driven looms for the weaving of cloth in large quantities, not to mention indoor running water and sewage disposal. There was an extensive reservoir system so that these activities could go on at any season.

All of this machinery is breaking down, or has broken down. The Kesset don't have the knowledge to repair it, and neither do most Ta'arane any more-- the artisan class was mostly wiped out with the fall of the cities. But the Kesset try to maintain the way of life they took over from the Ta'arane, and this requires a lot of human labor to be substituted for machine labor.

Iron

The Kesset also need slaves to work their iron mines. There are iron deposits in the valley, but the Ta'arane did not work iron nor mine it; they knew only bronze until the conquest. The Kesset brought ironworking technology from the steppes.

Somewhere "up there" (I'm vague about this geography, and don't expect it to become important), there's a mountainous area with extensive iron deposits and forests. In this area there are permanent settlements of Kesset-related peoples whose wealth is iron. One of the reasons the Kesset invaded the plain is that they identified iron deposits there.

Over the years, the Kesset have kidnapped or persuaded ironworkers from the mountains to settle in the Kesset cities (former Ta'arane cities). They have merged into Kesset society and brought their knowledge with them, including mining and the working of ore. The Kesset now have all the iron they need, and trade it to the steppe tribes (including the Locust People). Kesset ironworks are also contributing significantly to deforestation of the plain (which is mostly savanna anyway) and surrounding hills (except on the Woneiyal side).

For the Ta'arane slaves, being sent to the iron mines is a terrible fate. Working conditions in the mines are hellish and unsafe. Also the Ta'arane regard iron with a superstitious fear, as one of the causes of their downfall.